The present disclosure generally relates to medical ultrasound imaging, and, more particularly, to methods and apparatus for artifact elimination in time-gated anatomical imaging.
Imaging of internal anatomical structures by ultrasound, MRI, and CT requires sequential acquisition of data from different portions of the volume to be imaged. Often these acquisitions can be completed in a time that is short enough that anatomical motions are captured clearly or are unimportant. In other cases the acquisition time is long enough that significant anatomical motion occurs, distorting or obscuring the desired image.
When the motion is periodic in time, for example, as in a cardiac or respiratory cycle, it is common practice to split the acquisition into smaller sub-volumes, each of which is acquired in a sufficiently short time to provide a good image. These sub-volumes are time-synchronized with the same point in different cycles of the periodic motion and then, after acquisition, are combined into a single image of the desired volume. Often, especially in ultrasound imaging, multiple images of each sub-volume are obtained so as to build a moving picture of the entire volume over the time of an entire cycle of the motion.
It would be desirable to obtain the entire image using as few sub-volumes as possible because this minimizes the number of cycles of the motion, e.g. heartbeats, and therefore, the time required for data acquisition. Shorter acquisition times are easier for the patient and reduce the likelihood of artifacts due to extraneous motion of the patient or the imaging equipment or due to departures from perfect repeatability of the anatomical motion over multiple cycles.
Unfortunately, using fewer sub-volumes requires that they each be larger if they are to encompass the same volume of interest, thereby increasing the acquisition time for each sub-volume. If the anatomical motion is fast enough, the amount of motion during the acquisition time of a single sub-volume will be large enough to cause image artifacts at the boundaries between sub-volumes. For example, in volumetric imaging of the heart, data at the final edge of one subvolume would be acquired slightly later in the heartbeat than the data at the adjacent starting edge of the next sub-volume. A fast-moving portion of the heart wall crossing this boundary between sub-volumes would be shown in slightly different positions on either side of the boundary, resulting in a “tearing” artifact which will be visually objectionable and may cause difficulties in the reconstruction of the whole image.
In U.S. Pat. No. 5,993,390, a segmented 3-D cardiac ultrasound imaging apparatus builds a volumetric image of the heart by acquiring image data from several separate volume segments during separate cardiac cycles (heartbeats). The apparatus splices the images together to make a single volumetric image. The time within a cardiac cycle when the image data are acquired is referred to the cardiac phase, and an entire volume segment is imaged at the given phase in the cardiac cycle allocated to that volume segment.
However, when a fast-moving structure such as a heart valve spans a boundary between volume segments, a “tearing” artifact can sometimes occur. A probable cause may be due to the fact that even within a single cardiac phase, the time needed to acquire all of the data from a volume segment was sufficient for the fast-moving valve to be in a slightly different position in one volume segment in which it was imaged at the beginning of the phase, than in the adjacent volume segment in which it was imaged at the end of the phase. Accordingly, when the volume segment images are spliced together, the valve briefly appears “torn”.
Let us consider further the case of volumetric ultrasound imaging of the heart. Due to the speed of cardiac motions, it is desirable to obtain image data at as high a frame rate as possible. Images of the entire region of interest must be obtained as frequently as possible. Typically 15 Hz frame rates are barely usable; 30–60 Hz being preferable.
The amount of time required to obtain ultrasound data from a large volume encompassing most of the heart is long enough that an adequately high frame rate may not be possible. In such a case, the practice is to acquire the data in several smaller adjacent sub-volumes, each synchronized with a different beat of the cardiac cycle via an ECG signal. Then the sub-volumes are spliced together in the displayed image as if they were all from the same cycle. As illustrated in FIG. 1, each sub-volume consists of n 2-D scan planes with the order of scanning from 1 to n noted at the bottom for the first two sub-volumes (10, 12). The scan planes are oriented perpendicular to the page and stacked left to right. In each sub-volume, after a synchronization trigger from the ECG, the scan planes are acquired in the order indicated, so that like-numbered scan planes are acquired at essentially the same instant in different cardiac cycles.
The problem comes at the boundaries between sub-volumes. At the boundaries, the scan plane acquired at the end of the frame in one sub-volume (10) is displayed adjacent to the scan plane acquired at the beginning of the frame in the next sub-volume (12). A fast-moving part of the heart such as a valve will have moved enough, even during the relatively short frame time to be in different positions at the beginning and end of the frame time. This results in the “tearing” artifact 14 depicted in FIG. 1.
As a result, a single acquisition from a volume segment has been treated as if it were acquired instantaneously, but in fact is not. A small amount of time elapses between the first line acquired and the last (between the beginning and the end of the cardiac phase). This is not a problem within a single region, since so long as adjacent scan lines were acquired close in time to each other, the tissue imaged by those lines will all be in nearly perfect relative alignment. However, stacking multiple volume segments, with typical prior art scan patterns, together can cause image artifacts at the volume segment boundaries because the end of one volume segment's acquisition is spliced to the beginning of the next volume segment's acquisition.
FIG. 2 illustrates a schematic cross-sectional view of an imaged volume perpendicular to the scan lines. The locations of individual scan lines 16 and four (4) volume segments 18, 20, 22 and 24 are shown. The scan lines are numbered in the order of their acquisition; each line is designated with the volume segment to which it belongs (VSn) and the line number within that volume segment. In this example, there are only sixteen (16) rows and four (4) columns per volume segment, but typically there would be many more lines in a volume segment. A discontinuity at the boundary can be observed in FIG. 2, as indicated by reference numeral 26, there being a difference of forty eight (48) scan lines worth of time across the boundary. With more lines in a volume segment, this difference would be higher.
Accordingly, a method and apparatus for artifact elimination in time-gated anatomical imaging which overcomes the problems in the art discussed above would be desirable.